Handover (HO) is a procedure in which a mobile station (MS) moves from an air interface of one base station to an air interface of another base station. A handover procedure of a general IEEE 802.16e system is described below.
In an IEEE 802.16e network, a serving base station (SBS) may broadcast neighbor base station information through a neighbor advertisement (MOB_NBR-ADV) message in order to inform a mobile station (MS) of basic network configuration information (topology).
The MOB_NBR-ADV message includes system information of a serving base station and neighbor base stations, for example, preamble indexes, frequencies, handover (HO) optimization possibility, downlink channel descriptor (DCD)/uplink channel descriptor (UCD) information, etc.
The DCD/UCD information includes information required for an MS to perform information exchange in downlink and uplink. For example, the DCD/UCD information includes HO trigger information, a medium access control (MAC) version of a base station, Media Independent Handover (MIH) capability information, etc.
A general MOB_NBR-ADV message includes only information associated with neighbor base stations of an IEEE 802.16e type. Information associated with base stations of the type other than an IEEE 802.16e type may be broadcast to MSs through a service identity information advertisement (SII-ADV) message. Accordingly, an MS may acquire information associated with base stations of a heterogeneous network by requesting an SBS to transmit the SII-ADV message.
A procedure for performing handover by an MS, which has acquired information associated with neighbor base stations using the above-described method, in an IEEE 802.16e network is described below in more detail.
The general handover procedure in an IEEE 802.16e network may include HO initiation and preparation, HO execution, and HO completion.
An example of a basic handover procedure configured as described above is described below with reference to FIG. 1.
FIG. 1 illustrates an example of a handover procedure which may be performed in an IEEE 802.16e system.
As shown in FIG. 1, first, a mobile station (MS) may access a serving base station (SBS) to perform data exchange (S101).
The SBS may periodically broadcast information associated with neighbor base stations (BSs) located within the coverage of the SBS to the MS through an MOB_NBR-ADV message (S102).
The MS may begin to scan candidate HO BSs using an HO trigger condition while communicating with the SBS. The MS may transmit a handover request (MOB_MSHO-REQ) message to request the SBS to perform a handover procedure when a handover condition is satisfied, for example, when a predetermined hysteresis margin value is exceeded (S103).
The SBS may inform candidate HO BSs included in the MOB_MSHO-REQ message of the handover request received from the MS through an HO-REQ message (S104).
The candidate HO BSs may take precautions for the MS which has requested handover and transmit information associated with handover to the SBS through an HO-RSP message (S105).
The SBS may transmit the information associated with handover acquired from the candidate HO BSs through the HO-RSP message to the MS through a handover response (MOB_BSHO-RSP) message. The MOB_BSHO-RSP message may include information required to perform handover such as an action time for handover, a handover identifier (HO ID) and a dedicated HO CDMA ranging code (S106).
The MS may determine one target BS among the candidate HO BSs based on information included in the MOB_BSHO-RSP message received from the SBS. Then, the MS may attempt to perform ranging by transmitting a CDMA code to the determined target BS (S107).
The target BS which has received the CDMA code may transmit information indicating whether or not ranging has succeeded and physical correction values to the MS through a ranging response (RNG-RSP) message (S108).
Then, the MS may transmit a ranging request (RNG-REQ) message for authentication to the target BS (S109).
The target BS which has received the ranging request message of the MS may provide system information such as a connection identifier (CID) which can be used in the BS to the MS through a ranging response message (S110).
When the target BS has successfully completed authentication of the MS and has transmitted all update information, the target BS may notify the SBS of the MS of whether or not handover has succeeded through a handover completion message (HO-CMPT) (S111).
Thereafter, the MS may exchange information with the target BS which performs handover (S112).
The above description has been given assuming that the handover process is performed between the MS and the BS which follow the IEEE 802.16e standard (WirelessMAN-OFDMA Reference System). Hereinafter, in the present specification, a system to which a general technology including the IEEE 802.16e standard is applied is referred to as a “legacy system” for ease of explanation. An MS to which the legacy technology is applied is referred to as a “Yardstick MS (YMS)” or “legacy MS” and a BS to which the legacy technology is applied is referred to as a “Yardstick BS (YBS)” or a legacy BS”.
A system to which an advanced technology including the IEEE 802.16m standard (WirelessMAN-OFDMA Advanced System) is applied is referred to as an “advanced system”. An MS to which the advanced technology is applied is referred to as an “advanced MS (AMS)”, and a BS to which the advanced technology is applied is referred to as an “advanced BS (ABS)”.
The YBS has only a legacy zone (L zone or LZone) having a physical channel frame structure applied to a legacy system and the ABS may support only the AMS or both the AMS and the YMS. When the ABS supports only the AMS (WirelessMAN-OFDMA advanced system only), the ABS has only an advanced MS support zone (M zone or MZone) having a physical channel frame structure applied to an advanced system. When the ABS supports both the AMS and the YMS (WirelessMAN-OFDMA Reference System/WirelessMAN-OFDMA Advanced co-existing System legacy support), the ABS may operate in a mixed mode. In the following, the mixed mode operation is described with reference to the case in which the legacy system is an IEEE 802.16e system and the advanced system is an IEEE 802.16m system. When the ABS operates in the mixed mode, the ABS has both the legacy zone (LZone) and the advanced MS support zone (MZone). In the mixed mode, the uplink (UL) zone may be divided into an L zone and an M zone through Time Division Multiplexing (TDM) or Frequency Division Multiplexing (FDM).
The AMS may receive services from both the ABS and the YBS. That is, the AMS may receive a service through any one of the advanced MS support zone and the legacy zone and may perform both a handover procedure defined in the legacy system and a handover procedure defined in the advanced system. The mixed mode operation is described below assuming that the legacy system is an IEEE 802.16e system and the advanced system is an IEEE 802.16m system as mentioned above.
While the AMS receives a service from an M zone of an ABS that supports both the AMS and the YMS, the AMS may need to be handed over to the legacy system or to an L zone of another ABS. In this case, the AMS may perform network reentry to a target BS (a YBS or an L zone of an ABS that operates in the mixed mode) according to a network reentry procedure defined in the legacy system. However, before performing network reentry, the AMS may need to acquire information associated with neighbor legacy BSs or neighbor BSs that support the legacy system.